The present disclosure relates to semiconductor manufacturing, and more particularly, to plasma ashing processes for removing photoresist and post etch residues from a substrate.
Recently, much attention has been focused on developing low k dielectric thin films for use in the next generation of microelectronics. As integrated devices become smaller, the RC-delay time of signal propagation along interconnects becomes one of the dominant factors limiting overall chip speed. With the advent of copper technology, R (resistance) has been pushed to its practical lowest limit. As such, attention is focused on reducing C (capacitance) in order to increase overall chip speed. One way of accomplishing this task is to reduce the dielectric constant (often referred to as “k”) of the thin insulating films surrounding interconnects, thereby reducing C and improving overall chip speed.
Traditionally, silicon dioxide (SiO2) has been employed as an insulating film material. The terms low k and high k as used herein are relative to the dielectric constant (k) of silicon dioxide (SiO2), i.e., a low k material generally refers to a material having a dielectric constant less than that of silicon dioxide (e.g., less than about 3.9) and a high k material generally refers to a material having a dielectric constant greater than that of silicon dioxide (e.g., greater than about 3.9). Low k materials generally include, but are not limited to, organic polymers, amorphous fluorinated carbons, nanofoams, silicon based insulators comprising organic polymers, carbon doped oxides of silicon, fluorine doped oxides of silicon, and the like.
In fabricating integrated circuits on substrates (e.g., wafers), the substrates are generally subjected to many process steps before finished integrated circuits can be produced. Low k dielectric materials, especially carbon containing low k dielectric materials, can be sensitive to some of these process steps. For example, plasma used during an “ashing” step can strip both photoresist materials and remove a portion of the carbon containing low k dielectric film. Ashing generally refers to a plasma mediated stripping process by which residual photoresist and post etch residues are stripped or removed from a substrate upon exposure to the plasma. The ashing process generally occurs after an etching or implant process has been performed in which a photoresist material is used as a mask for etching a pattern into the underlying substrate or for selectively implanting ions into the exposed areas of the substrate. The ashing step is typically followed by a wet chemical treatment to remove traces of the etch residues. However, the wet chemical treatment can cause further degradation of the low k dielectric, loss of material, and can also cause an increase in the dielectric constant.
It is important to note that ashing processes significantly differ from etching processes. Although both processes can be plasma mediated, an etching process is markedly different in that the plasma chemistry is chosen to permanently transfer an image into the substrate by removing portions of the substrate surface through openings in a photoresist mask. The plasma generally includes high-energy ion bombardment at low temperatures (e.g., about room temperature (about 21° C.) to about 140° C.) and low pressures (of the order of millitorr) to remove portions of the substrate. Moreover, the portions of the substrate exposed to the ions are generally removed at a rate equal to or greater than the removal rate of the photoresist mask.
In contrast to etching processes, ashing processes generally refer to selectively removing the photoresist mask and any polymers or residues formed during etching. The ashing plasma chemistry is much less aggressive than etching chemistries and is generally chosen to remove the photoresist mask layer at a rate much greater than the removal rate of the underlying substrate. Moreover, most ashing processes heat the substrate to temperatures greater than 200° C. to increase the plasma reactivity, and are performed at relatively higher pressures (of the order of a torr). Thus, etching and ashing processes are directed to removal of significantly different materials and as such, employ completely different plasma chemistries and processes. Successful ashing processes are not used to permanently transfer an image into the substrate. Rather, successful ashing processes are defined by the photoresist, polymer, and residue removal rates without affecting or removing underlying layers, e.g., low k dielectric layers.
Studies have suggested that a significant contribution to low k dielectric degradation during photoresist removal processes results from the use of oxygen, nitrogen, and/or fluorine containing gas sources for generating ashing plasmas. Although gas mixtures comprising these sources efficiently ash photoresist from the substrate, the use of these gas sources has proven detrimental to substrates containing low k dielectrics. For example, oxygen-containing plasmas can raise the dielectric constant of low k dielectric underlayers during plasma processing. The increases in dielectric constant affects, among others things, interconnect capacitance, which directly impacts device performance. Moreover, the use of oxygen-containing plasmas is generally less preferred for advanced device fabrication employing copper metal layers, since copper metal is readily oxidized.
Ideally, the ashing plasma should not affect the underlying low k dielectric layers and should preferentially remove only the photoresist material. The use of traditional dielectrics such as SiO2 provided high selectivity with these gas sources (e.g., oxygen, nitrogen, and/or fluorine containing gas sources) and was suitable for earlier device generation. An issue with low k dielectrics is their sensitivity to attack by oxidative plasma species. In order to minimize damage to the low k dielectric, essentially oxygen free (e.g., comprising less than about 20 parts per million (ppm) oxygen (O2)) and essentially nitrogen free (e.g., comprising less than about 20 ppm nitrogen (N2)) plasma processes have been developed. One such process is described in U.S. Pat. No. 6,630,406 to Waldfried et al., wherein the process includes generating plasma from a gas mixture comprising a noble gas (e.g., helium) and hydrogen. The oxygen free and nitrogen free plasma, such as the above noted plasma formed from helium and hydrogen, is less aggressive and does not completely react with the photoresist in the traditional sense. Rather, it is believed that the plasma renders portions of the photoresist to be removable such as by sublimation and volatilization. As a result, while essentially oxygen and nitrogen free plasmas are effective for removing photoresist material from the substrate, the plasma exposure tends to deposit large bodies of the sublimed (or volatized) or removed photoresist and byproducts within the processing chamber and in areas downstream from the plasma process chamber such as in the exhaust lines and any components therein. As a result, periodic cleaning of the process chamber is required, which generally requires the use of oxidizing plasma. The oxidizing plasma can provide a means for in situ cleaning of the chamber. However, the oxidizing plasma has been found to cause the baffle plate assembly (commonly employed in the process chamber to uniformly distribute the ashing plasma to the substrate) to increase in temperature, especially at the impingement center where the plasma first impacts the baffle plate assembly. The increase in temperature during subsequent wafer processing has been found to decrease the ashing rate and negatively impact the uniformity of the ashing process across the wafer surface, particularly the removal rate difference between the center and edge of the wafer.
An additional problem with oxygen free and nitrogen free plasmas is the non-uniformity of the plasma exposure. Since these plasmas are less aggressive, non-uniformity is a significant issue. Some downstream plasma ashers have a narrow diameter orifice plasma tube in which the plasma is generated. The diameter of the substrate is generally much larger than the diameter of the plasma tube orifice. As such, a baffle plate(s) is typically positioned near the plasma tube outlet to deflect the plasma as it enters the process chamber such that the species in the plasma are uniformly dispersed across the substrate. The temperature of the baffle plate assembly can undesirably vary during normal machine operation depending on the duration of plasma exposure per wafer, the number of wafers processed in a batch and the length of time between batches. As a result, the ashing rate and uniformity can be affected in substantially the same way (although to a lesser extent) as was described previously for the oxidizing plasma cleaning process.
Accordingly, there is a need in the art for a process for reducing the temperature at the impingement portion i.e., center portion, of the baffle plate, while maintaining or enhancing the photoresist removal rate.